hv_netvsc: rework link status change handling

There are several issues in hv_netvsc driver with regards to link status
change handling:
- RNDIS_STATUS_NETWORK_CHANGE results in calling userspace helper doing
  '/etc/init.d/network restart' and this is inappropriate and broken for
  many reasons.
- link_watch infrastructure only sends one notification per second and
  in case of e.g. paired disconnect/connect events we get only one
  notification with last status. This makes it impossible to handle such
  situations in userspace.

Redo link status changes handling in the following way:
- Create a list of reconfig events in network device context.
- On a reconfig event add it to the list of events and schedule
  netvsc_link_change().
- In netvsc_link_change() ensure 2-second delay between link status
  changes.
- Handle RNDIS_STATUS_NETWORK_CHANGE as a paired disconnect/connect event.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>